Fermented milks, such as fermented milk drinks, lactic acid bacteria beverages, yoghurt, cultured milks and cheese, are often produced by providing animal milks, such as cow milk, goat milk, horse milk and the like, as culture media and fermenting them with lactic acid bacteria. However, such lactic acid bacteria generally have strict auxotrophy, and many strains thereof do not grow well in culture media composed solely of animal milks. Even with bacterial strains having relatively good proliferability, culture media composed of animal milks alone are considered to require continued cultivation over several days if fermented milk ingredients having acidity sufficient for use in the production of fermented milks are desired.
In the cultivation for the production of a fermented milk in which importance is placed on the viable cell count of lactic acid bacteria, long cultivation however gives rise to another problem in that it leads to a reduction in the viable cell count of the lactic acid bacteria. For example, fermented milks making use of fermented milk of the viable cell type, such as yoghurt, are extensively consumed as health-promoting foods having physiological effects such as intestinal function controlling effect and immunopotentiating effect. For the maintenance of these physiological effects at high levels, it is important to retain useful bacteria, such as lactic acid bacteria, in as high viable cell count as possible in a viable state and further to keep high the activity (acid producing ability). On the other hand, the flavor of a fermentation product is of importance for a fermented milk. This makes it impossible to choose a bacterial strain from the viewpoint of proliferability alone, and on the contrary, a bacterial strain may have to be selected for its ability to give fermentation products of good flavor despite its poor proliferability.
In the cultivation of lactic acid bacteria, it is therefore common practice to add one or more of various growth promoting substances to a culture medium in order to improve the efficiency of the cultivation. Currently known examples of growth promoting substances or those confirmed to be effective for the promotion of growth include Chlorella extract, iron salts, vitamins, proteolysates containing amino acids and peptides, and yeast extract. They are used for the above-mentioned purpose.
For the retention of the usefulness of lactic acid bacteria, it is necessary not only to promote its growth but also to inhibit death of its cells and further, is required to maintain a high viable cell count in the final product during storage. A marked reduction is observed in the viability of lactic acid bacteria especially when a low-fat fermented milk such as low-fat yoghurt is produced using skim milk powder or when lactic acid fermentation proceeds excessively. This problem becomes more serious when low-calorie fermented milks or low-pH fermented milks are produced. With a view to maintaining such viable cell counts, substances such as Chlorella are added these days.
The addition of such substances, however, often affects the flavors of products themselves and moreover, involves a problem in that the costs of the products are increased. In addition, these substances can hardly maintain the high activities of such lactic acid bacteria although they may be able to keep high viable cell counts.
An object of the present invention is therefore to find out a novel growth-promoting or viability-improving substance which is free of a problem with flavor and, when simply added, can increase the viable cell count of lactic acid bacteria and can also maintain the viable cell count in the final product, and further to provide a fermented milk which makes use of the substance to maintain as many cells of the lactic acid bacteria as possible in a viable state and also to keep high the activity (acid producing ability) of the cells.